1. Field of the Invention
The present invention relates to a method for preparing an anode electrode for a high voltage electrolytic capacitor, and more particularly to a method for preparing an anode electrode for a high voltage electrolytic capacitor, in which an oxide film having a uniform thickness and a large surface area is formed on an aluminum electrode to be used as the anode electrode.
2. Description of the Related Art
Generally, an aluminum electrolytic capacitor uses an aluminum foil for an anode material thereof. In such an aluminum electrolytic capacitor, an oxide film is formed, as a dielectric, on the surface of the anode. An organic solvent or solid electrolyte is used as an electrolyte to be provided between electrodes. In order to achieve an increase in capacitance, the dielectric of such an electrolytic capacitor should have a high dielectric constant, a large surface area, and a small thickness.
Typically, conventional methods for preparing an aluminum foil for an electrolytic capacitor involve an etching process for roughing the surface of an aluminum foil having a high purity of 99.9% or more, thereby achieving an increase in the surface area of the aluminum foil, and a formation process for forming a dielectric film on the aluminum foil. In such conventional methods, accordingly, the above mentioned increase in capacitance is achieved by conducting an electrochemical or chemical etching process for the aluminum foil under the condition in which an appropriate element is added to the material of the aluminum foil, or under the condition in which appropriate ions are added to a solution used in the etching process in order to rough the surface of the aluminum foil.
All known methods of increasing the surface of an aluminum electrode used for electrolytic capacitors involve a process for etching the surface of an aluminum foil used for the aluminum electrode prior to the formation of an oxide film on the aluminum foil. In order to rough the surface of the aluminum foil, addition of sulfate ions serving to increase the etchability of the aluminum foil has also been made. However, where an oxide film is formed on the aluminum foil etched using sulfate ions, there is a problem in that it is difficult to expect a desired surface area increase because fine tunnels formed by those sulfate ions are blocked during the formation of the oxide film. Furthermore, the formation of the oxide film must be conducted for a lengthened period of time. For this reason, there is an increase in the moisture content of the oxide film. This requires a lengthened period of time for a dehydration.
U.S. Pat. No. 3,316,164 discloses a method for preparing an aluminum foil having a surface area increased from an initial surface area by 20 times in accordance with an addition of sulfate ions to an NaCl solution. In this method, however, the sulfate ions are absorbed onto the entire surface of the aluminum foil during an etching process, thereby producing a deposit of aluminum hydroxide sulfide on a cathode. Also, U.S. Pat. No. 5,062,025 discloses a method for manufacturing an electrolyte capacitor, in which filaments of aluminum and those of other metals are used in a combined fashion in order to achieve an increase in surface area. The capacitance increased in accordance with an increase in surface area is measured using a well-known universal impedance bridge. However, this method is different from the present invention in terms of the configuration.
The present invention has been made in view of the above mentioned problems involved in the related art, and an object of the invention is to provide a method for preparing an anode electrode for a high voltage electrolytic capacitor, which is capable of requiring a relatively reduced anodization time, effectively generating a reaction for the formation of tunnels, and maximizing the effect of sulfate ions added, thereby forming an oxide film having a uniform thickness and a large surface area.
In accordance with the present invention, this object is accomplished by providing a method for preparing an anode electrode of a high voltage electrolytic capacitor, comprising the steps of: anodizing an aluminum foil in a boric acid solution, thereby forming an oxide film of a uniform thickness on the aluminum foil; partially etching the aluminum foil, formed with the oxide film, in an NaCl solution containing sulfate ions; and re-anodizing the etched aluminum foil, thereby forming again an oxide film on the etched aluminum foil.